1. Field of the Invention
The present invention generally relates to semiconductor memory devices, and more particularly, to flash memory devices.
2. Discussion of Related Art
In general, a memory cell of a flash memory device can be classified into a Single-Level Cell (hereinafter, referred to as “SLC”) and a Multi-Level Cell (hereinafter, referred to as “MLC”) depending on the number of data bits stored. 1-bit data having a logical value “1” or “0” can be stored in the SLM. 2-bit data having a logical value of any one of “11”, “10”, “01”, and “00” can be stored in the MLC. Accordingly, after the program operation, the threshold voltage of the memory cell (SLC or MLC) included in the flash memory device is changed to a voltage corresponding to a stored data value.
For example, in the case where the flash memory device includes the SLC, the threshold voltage of the SLC is changed to a voltage corresponding to “1” or “0” after the program operation of the flash memory device. Meanwhile, in the case where the flash memory device includes the MLC, the threshold voltage is changed to a voltage corresponding to any one of “11”, “10”, “01”, and “00” after the program operation of the flash memory device.
The threshold voltage distributions of the memory cells according to the program procedure of the flash memory device in the related art will be described in more detail below with reference to FIG. 1.
FIGS. 1A to 1C are graphs illustrating the threshold voltage distributions of memory cells according to the program procedure of the flash memory device in the related art. FIG. 1 is related to the program operation of the flash memory device including the SLC. The graphs of FIGS. 1A to 1C do not illustrate actual threshold voltage distributions of the SLC, but illustrate threshold voltage distributions of the SLC, which are seen from the viewpoint of the voltage generator.
FIG. 1A is a graph illustrating the threshold voltage distributions of the SLC when the program operation of the flash memory device is performed at a cold temperature. FIG. 1b is a graph illustrating the threshold voltage distributions of the SLC when the program operation of the flash memory device is performed at a room temperature. FIG. 1c is a graph illustrating the threshold voltage distributions of the SLC when the program operation of the flash memory device is performed at a hot temperature.
As can be seen from FIGS. 1A to 1C, the threshold voltage of the SLC programmed at the cold temperature is relatively lower than that of the SLC programmed at the hot temperature. In other words, it can be seen that the locations of graphs PS31 to PS33 shown in FIG. 1C are slightly moved in a direction in which the voltage is increased (to the right side in FIG. 1C) compared with the locations of graphs PS11 to PS13 shown in FIG. 1A.
The reason why the threshold voltage distributions of the SLC look changed according to the temperature during the program operation as described above is that the operating conditions of internal circuits are varied depending on variation in temperature. The operating conditions of the internal circuits may include, for example, an amount of current of memory cells, the impedance of a word line when viewing the word line from the voltage generator, and threshold voltages and saturation currents of transistors included in a page buffer.
The operating conditions of the internal circuits decrease the voltage transferred to the gate of the SLC through the word line at the cold temperature. Furthermore, the operating conditions of the internal circuits increase the voltage transferred to the gate of the SLC at the hot temperature. Due to this, although the voltage generator applies a constant verify voltage to the word line, a verify voltage PV transferred to the gate of the SLC may be increased or decreased depending on the operating conditions of the internal circuits.
For example, if the verify voltage PV transferred to the gate of the SLC is decreased by the operating conditions of the internal circuits, the program operation is finished with the SLC not being sufficiently programmed. As a result, the whole threshold voltage of the programmed SLC is decreased (i.e., moved toward the left side in FIG. 1A), as in the graphs PS11 to PS13 of FIG. 1A. On the other hand, if the verify voltage PV transferred to the gate of the SLC is increased by the operating conditions of the internal circuits, the program operation is finished with the SLC being excessively programmed. As a result, the whole threshold voltage of the programmed SLC is increased (i.e., moved toward the right side in FIG. 1C), as in the graphs PS31 to PS33 of FIG. 1C.
Consequently, variation in the operating conditions of the internal circuits depending on the temperature during the program operation changes the verify voltage transferred to the gate of the SLC, and variation in the verify voltage changes the threshold voltage distributions of the SLC. As a result, the width of the threshold voltage distributions of the programmed SLC after the program operation must be narrowed as indicated by “W1”, but is widened as indicated by “W2” considering variation in temperature upon program.
If the width of the threshold voltage distributions of the SLC is widened as described above, the program operating speed of the flash memory device is decreased and a program disturbance phenomenon is generated.
In a similar way, the operating conditions of the internal circuits are changed depending on variation in temperature during the normal read operation. This will be described in more detail below.
When the normal read operation of the flash memory device is performed at the cold temperature, the operating conditions of the internal circuits are changed to decrease a read voltage RV that is actually transferred to the gate of the SLC. In other words, although the voltage generator applies a constant read voltage RV to the word line regardless of the temperature, the read voltage RV transferred to the gate of the SLC is decreased more at the cold temperature than at room temperature due to the operating conditions of the internal circuits. As a result, from the viewpoint of the voltage generator, the whole threshold voltage of the SLC during the normal read operation at the cold temperature may look relatively high compared with the whole threshold voltage of the SLC during the normal read operation at room temperature.
In a similar way, when the normal read operation of the flash memory device is performed at the hot temperature, the operating conditions of the internal circuits are changed to increase the read voltage RV that is actually transferred to the gate of the SLC. In other words, although the voltage generator supplies a constant read voltage RV to the word line regardless of the temperature, the read voltage RV transferred to the gate of the SLC is increased more at the hot temperature than at room temperature due to the operating conditions of the internal circuits that are changed depending on a temperature. As a result, from the viewpoint of the voltage generator, the whole threshold voltage of the SLC during the normal read operation at the hot temperature may look relatively low compared with the whole threshold voltage of the SLC during the normal read operation at room temperature.
Therefore, the threshold voltage distributions of the SLC during the normal read operation can be represented by the graphs PS11 to PS13, PS21 to PS23, and PS31 to PS33 as shown in FIGS. 1a to 1c. 
In more detail, the graphs PS11 to PS13 indicate the threshold voltage distributions of the SLC, respectively, which are taken into consideration from the viewpoint of the voltage generator when the data of the SLC programmed at the cold temperature are read at the cold temperature, room temperature, and the hot temperature, respectively, (i.e., during the normal read operation). Furthermore, the graphs PS21 to PS23 indicate the threshold voltage distributions of the SLC, respectively, which are considered from the viewpoint of the voltage generator when the data of the SLC programmed at room temperature are read at the cold temperature, room temperature, and the hot temperature, respectively. Furthermore, the graphs PS31 to PS33 indicate the threshold voltage distributions of the SLC, respectively, which are taken into consideration from the viewpoint of the voltage generator when the data of the SLC programmed at the hot temperature are read at the cold temperature, room temperature, and the hot temperature, respectively.
In the case where the threshold voltage distributions of the SLC are changed depending on a temperature during the normal read operation as described above, it is preferred that the width of the threshold voltage distributions of the SLC be narrow as indicated by “W1”. However, the width of the threshold voltage distributions of the SLC is widened as indicated by “W2”.
As described above, the threshold voltage distributions of the SLC are changed depending on variation in a temperature during the program operation and the normal read operation. However, the related art voltage generator applies a constant verify voltage PV or a constant read voltage RV to the word line regardless of a temperature. Due to this, the width of the threshold voltage distributions of a programmed SLC is further widened and failure may occur in the read operation.
In more detail, in the event that the data of a SLC programmed at the cold temperature are read at the hot temperature (corresponding to the graph PS13), a data value “1” corresponding to an erased SLC is read as indicated by “A” in spite of the programmed SLC, leading to failure in the read operation. As a result, if a constant read voltage RV is applied to the word line regardless of a temperature, the sensing margin of read data during the normal read operation is decreased. For example, the sensing margin of read data of a SLC having threshold voltage distributions as indicated by the graph PS13 of the graphs PS11 to PS13 is the lowest.
On the other hand, in the case where the flash memory device includes the MLC, the threshold voltage distributions of the MLC are widened and the sensing margin of read data is decreased, depending on variation in temperature during the program operation (more particularly, during the program verification operation) or during the read operation in a similar way as the above.